Willow warbler (Phylloscopus trochilus) is the most abundant bird in Varrio Nature Reserve, as well as in whole Finland. In Varrio Willow warblers are regularly observed in three point count monitoring (the graph in left, inner points data) and three block count monitoring lines (the graph in right). The annual monitoring time is around 20 June.

In pointwise monitoring willow warblers have been counted altogether 765 times (over 24 years, inner points data). In blockwise monitoring the number of counts was 1159 (over 19 years). In neither case is the trend of change statistically significant. Thus Varrio Willow warblers do not indicate any change in the population, environment or climate.

Calculated from regression lines Willow warbler is recorded at 2008 altogether 86 (85.61) times per year. This will be set to correspond population density for Willow warbler of 3000 pairs in the whole Varrio Nature Reserve. This density is based on National Bird Atlas data of Finland to ecological and geographical area of Varrio.